Such a piston is described in EP 0 385 390 A1 and usually comprises shaft surfaces which in some of the regions along the height of the piston shaft, are completely cylindrical and have a circular cross-section. The piston shaft is largely cylindrical in an upper region in order to close the chamber for the combustion that takes place inside an internal combustion engine. To reduce the piston weight, the pin hubs for receiving the conrod pin are inwardly set back in the direction of the piston shaft diameter. As a result, those regions of the pin hubs which are indispensable for receiving the gudgeon pin are offset from the shaft surface, i.e. inwardly from the piston's outer surface, relative to the piston axis. In consequence, material can be saved in the outer regions, thereby reducing the weight.
At the height of the pin hubs and below them, however, such a piston with inwardly offset pin hubs also has to comprise shaft wall portions, with which the piston makes contact with the cylinder wall once it has been installed into the cylinder of an internal combustion engine. These shaft wall portions, also designated as load-bearing shaft wall portions because they ensure that the piston is guided within the cylinder, are therefore formed in a lower region of the piston shaft at radially opposite sides relative to the pin axis, viz. at the thrust side on the one hand and at the counterthrust side on the other. These shaft wall portions ensure that the piston is supported at this sites during the reciprocating movement inside the cylinder; during this movement, the piston can tilt around the gudgeon pin axis. During this tilting movement at the top and bottom dead centers, there particularly occurs an alternation with respect to that side with which the piston makes contact with the cylinder wall, from the thrust side to the counterthrust side or vice versa. This tilting movement is restricted, as far as necessary, by the load-bearing shaft wall portions. In its lower region, the piston shaft of the pistons according to the class comprises communicating walls which are inwardly set back and in which the pin hubs are located. These communicating walls extend as far as the lower edge of the piston shaft. A wrap-around ring adapted so as to connect the two opposite shaft wall portions together is therefore missing at the piston's lower edge.
By economizing on material in the areas outside the set-back pin hubs, such a piston ensures that it is possible to reduce the piston weight considerably.